Found an even better article a few years later, again:
The best way to completely clear the rim jets is to plug each jet with plumbers putty, pour acidic toilet cleaner into the overflow tube located in the toilet tank, and let the acidic cleaner sit for a while and dissolve the build-up and deposits. After a few hours, remove the plumbers putty and flush several times. Finish the job by scrubbing with a brush to remove any leftover debris.
I just happened across this comprehensive answer a few years later. Here's the content:
You can test the toilet by pouring a 2-gallon bucket of water directly into the bowl. If the water swirls and goes down and stays down, then the jets are probably clogged.
You can repair this condition yourself with a little effort and a $20 bill. You will need a gallon of calcium, lime and rust remover, sold under the brand name CLR (at most home stores), a 1-quart soft-plastic container (metal might scratch the ceramic inside the bowl), a plastic bucket, rubber gloves, eye protection and a well-ventilated room. Use a portable fan if you have to. Wear a long-sleeved shirt and pants to protect your skin from the chemicals. Read and follow the safety directions on the CLR container.
First, turn off the water supply to the toilet. There should be a shut-off valve on the wall or floor under the tank. If not, turn off the water supply to the home. Flush the toilet, then drain and sponge out as much of the water as you can find from both the tank and the bowl.
Next, remove the tank top and the rubber flapper valve at the bottom of the tank. Start the cleanup by pouring all the CLR into the tank. It will fill the bowl as it enters and starts to clean the jets. Using the plastic container, scoop up the CLR from the bowl and pour it back into the tank.
Keep scooping and pouring, and you will start to see flecks of mineral deposits inside the bowl. At some point, the CLR may start to foam as the chemicals react with the calcium and lime, and you may notice an acidic odor.
Leave the room or use the fan and venting to avoid breathing the fumes. When the air has cleared, continue to scoop and pour until you see the CLR running out of the jets freely.